Three color high resolution displays based on spatial light modulators (SLMs), require three SLMs to produce an image. Each SLM operates on one of the three colors: red (R), green (G) and blue (B). The resulting full color image is a superposition of the individual red, green and blue images on the screen. Liquid crystal (LC) light valves are one known type of SLM.
A typical prior art configuration for a three color scheme is shown in FIG. 1. Specifically, FIG. 1 shows a prior art configuration 10 which uses three transmission type light valves (or SLMs) 12, 14, 16; one for each color. The operating principle is based on rotation of polarization of incoming light for each pixel. An image is formed in a transmission liquid crystal light valve when the light valve is placed between polarizers. In a projection system, each of the three light valves controls the image formation of one of the three individual color components. The complete projection system typically uses a white lamp arc source 18 followed by dichroic color filters 20A and 20B which split the incoming white light into red (R), green (G) and blue (B) components. Each color component is directed to one of the three light valves. A polarizer 22 is placed at the input to each light valve to select only one polarization state to enter the light valve. A second polarizer 24 is placed after each of the three light valves to select the image forming light. An X-cube 26 is typically used to combine the images produced by the individual red, green and blue light valves to form the full composite image. A projection lens 28 completes the system to magnify and project the images onto a screen.
A number of projection systems using transmission light valves are commercially available. However, in order to prepare light valves with a higher number of pixels as required for high resolution displays, the liquid crystal panel becomes large. It is difficult to produce transmission light valves with very small pixels since the electronic circuity needed for operation of the light valves obscures the light passage through the pixels to an intolerable degree. Instead, reflection based light valves are emerging for high resolution applications. In the reflection mode, it becomes possible to fabricate the mirror structure directly above the electronic circuitry. This mode allows smaller pixel area without obstructing the light passage caused by the electronic circuitry, thus allowing maximum light throughput.
The above described configuration as well as other projection displays employing reflective liquid crystal light valves utilize an optical arrangement similar to that shown in FIG. 2. Light from an illumination system (not shown) is divided into beams of red (R), green (G) and blue (B) by means of dichroic mirrors (also not shown in FIG. 2). Each colored beam is directed to its corresponding polarizing beam splitter (PBS) 42, 44 and 46 adjacent to a reflective light valve (LV) 48, 50 and 52. Illustratively, PBS 42 and LV 48 are selective for red light, PBS 44 and LV 50 are selective for green light and PBS 46 and LV 52 are selective for blue light. Each polarizing beam splitter directs a polarized beam onto its light valve which in its dark state typically acts similarly to a mirror. Moreover, each polarizing beam splitter contains a dichroic coating which is selective for reflecting a specific color of light. In FIG. 2, PBS 42 contains a dichroic coating specific for red (R'), PBS 44 contains a dichroic coating specific for green (G') and PBS 46 contains a dichroic coating specific for blue (B').
The incident light is reflected back into the polarizing beam splitter without change in polarization and is therefore reflected again at the PBS back toward the light source. If a voltage is applied across the liquid crystal layer there is a rotation in polarization and the reflected beam is transmitted by the polarizing beam splitter and enters a so called X-cube 54.
X-cube 54 consists of 4 right angle prisms to which dichroic coatings have been applied, cemented together to form a cube. The cube has the property that it contains two color specific reflecting planes within it by careful alignment of the four prisms from which it is constructed. Typically one coating will reflect only red (R) and the other only blue (B). Green (G) light in this case would be transmitted by both. In the drawings, the dichroic coating selective for red is labeled (R') whereas the dichroic coating selective for blue is labeled (B'). In practice, however, the dichroic coatings may not be perfect, that is the blue (B) coating may actually reflect a small amount of red (R) light and this can give rise to unexpected and unwanted additional light paths within the prism.
Red (R) light entering the X-cube is reflected by a red reflecting dichroic coating (R') into a projection lens not shown. Similarly a beam of another color, for example blue, entering its appropriate face of the X-cube is reflected into a common beam entering the projection lens. That is, the X-cube acts as a color combining element that redirects red, green and blue light from the three modulating light valves into a common beam that enters the projection lens for imaging on a distant screen.
A major problem with prior art X-cubes of the type shown in FIG. 2 is the unwanted reflections which occur between the various dichroic coatings on the cube. This problem is illustrated in FIGS. 3(a)-(b). Specifically, light from a red cell has to traverse a blue reflecting coating (B') to meet the red reflecting coating (R') that directs the beam to the projection lens. A small amount of red light indicated by the dashed line, is reflected by the blue coating (B') as shown, toward the red reflecting coating (R') where it is redirected to light valve 48 and subsequently gives rise to a new unwanted beam (dashed line) that can reach the projection lens.
Even a small amount of light reflected in this way can give rise to an unfocused background illumination that raises the light level in otherwise dark regions appreciably reducing contrast. A similar effect can occur in other channels as shown for the straight through channel in FIG. 4. The above-mentioned effect can play an important part in lowering the image contrast as measured by comparing the light produced from a bright region on the light valve to the light produced from a dark region on the light valve. These bright and dark regions are produced by electronically driving regions of the light valve completely on while other regions of the light valve are driven completely off. Specifically, a much lower contrast is observed when the light valves are turned on in a checkerboard fashion as in the American National Standards for Audiovisual Systems (ANSI) contrast measurement, since in this case light from the bright portions of the checkerboard can reach dark regions by means of spurious reflections.
FIGS. 3(a)-(b) and 4 show that light which is in a direction normal to the light valve results in the spurious reflections. In an actual optical projection system, a cone of light is incident onto the light valve. This case also leads to the same spurious reflections. Since the internal angles of the X-cube are at 90.degree., any light with a principal entry ray incident to an internal X-cube surface at approximately 45.degree. (e.g. 45.+-.10.degree.) will also be reflected along its original direction back to the light valve (45.degree. reference orientation). This is a result of the 90.degree. corner cube effect produced by the two internal interfaces. As is well known in the art, two reflecting surfaces aligned at 90.degree. will always send light back along its original direction. Thus, all light within a specific cone angle will be retro-reflected also within the same cone angle. This leads to the same spurious reflections depicted in FIGS. 3(a)-(b) and 4 within the entire light cone angle.
In view of the drawbacks mentioned above with prior art projection displays there is a continued need to develop a new and improved optical projection system which provides superior resolution and enhanced contrast imaging.